A desirable component for a mobile phone is a switching power supply such as a direct current to direct current (DC-to-DC) converter that transfers energy from a source to a load. Typically, DC-to-DC converters offer much greater efficiencies than do linear voltage regulators while regulating power being transferred to a load such as the circuitry of a mobile phone. However, linear voltage regulators at present offer an advantage by generating much less spectral noise than DC-to-DC converters. Moreover, incorporating DC-to-DC converters into mobile phone circuitry is made more problematic by third-generation (3G) and later mobile phone standards, which are increasingly restrictive with regard to spurious radio frequency (RF) transmissions. As a result of a need to reduce spurious RF transmissions from mobile phones, any leakage of DC-to-DC converter switching noise into a mobile phone's transmitter circuitry is preferably reduced. Traditional attempts to reduce switching noise leakage have resulted in either prohibitively expensive filtering components or unacceptable results when mobile phone emission standards are applied.
Making the switching noise of a DC-to-DC converter less periodic by a frequency dithering of the DC-to-DC converter's switching signal is helpful. However, unlike a frequency dithering scheme for a crystal oscillator that provides acceptable electromagnetic interference (EMI) performance, a frequency dithering of a DC-to-DC converter's switching signal often results in an unacceptable increase in output voltage ripple. One reason for the increase in output voltage ripple is that a fundamental criterion for steady state operation of a DC-to-DC converter is violated by using frequency dithering.
Typical DC-to-DC converters have two distinct phases of operation. A first phase occurs when energy is stored in an inductor's magnetic field, and a second phase occurs when the stored energy is transferred to a load. An imbalance of energy transfer will often result in either an energy buildup in the inductor's magnetic field or a collapse of the inductor's magnetic field. As a result, undesirable output voltage variations will likely occur as an inductor current creating the inductor's magnetic field is integrated by one or more output filter capacitors. A resulting output voltage ripple will increase as a charge transfer to the filter capacitors becomes imbalanced. As illustrated in FIG. 1, the inductor current will show an imbalance as a pulse width modulated (PWM) switching signal is stepped from one period to another. In other words, the changes in switching signal frequency may introduce a variation of the inductor average current, which in turn results in an undesirable output voltage ripple. In FIG. 1, the switching signal labeled “PWM” may also be referred to as the “Lnode” signal. The “Lnode” signal is located on a node of an inductor that is either driven low or high by one or more power Field Effect Transistors (FETs) making up a part of a DC-to-DC converter's circuitry.
A challenge is how to change the frequency of a DC-to-DC converter without introducing a charge imbalance. FIG. 2 illustrates an adjusted inductor current resulting from an ideal PWM waveform. The ideal PWM waveform, shown in FIG. 2, and a corresponding flywheel voltage, will provide a constant average current independently of a frequency deviation. A plurality of transitional periods (T1, T2, and T3) may be calculated. Moreover, an output voltage ripple can be reduced to the integral of an inductor ripple current. A first order linear design approach based on inductor current is preferably used to simplify an implementation of a DC-to-DC converter having reduced output voltage ripple.
A transitional period required when switching between a switching frequency Fn and another switching frequency Fn+1 may be defined as:
                                          T            Transitional                    =                                                    (                                                      T                                          F                      ⁡                                              (                        n                        )                                                                              +                                      T                                          F                      ⁡                                              (                                                  n                          +                          1                                                )                                                                                            )                            *                              (                                  1                  -                  D                                )                                      2                          ,                            Eq        .                                  ⁢        1            where D is the Duty cycle, and TF(n)+TF(n+1) are the periods of the two frequencies Fn and Fn+1.
Equation 1 above may appear to be an over-simplification, but the generation of a PWM signal is as simple as comparing an error signal with a ramp using a voltage comparator. A fast dithering method of generating a PWM switching signal that allows the PWM switching signal to be changed every cycle without introducing a significant output ripple voltage is disclosed in co-pending patent application Ser. No. 11/756,909, now U.S. Pat. No. 7,928,712, filed Jun. 1, 2007.
Spurious RF transmissions due to leakage of DC-to-DC converter switching noise into a mobile phone's transmitter circuitry may be reduced somewhat by a fast dithering of a DC-to-DC converter's switching signal. The fast dithering of a switching signal is a two-dimensional process that spreads a plurality of switching frequency spurs over a wide range while improving output voltage ripple rejection. While fast dithering of a switching signal provides good a design starting point it has been discovered that every data burst (FIG. 3) from a Global System for Mobile communication (GSM) mobile phone will have very similar signatures. As a result, fast dithering may result in a poor quality communications link as explained below.
In GSM, the quality of the link is measured in terms of bit error rate (BER) and a frame erasure rate (FER) for various traffic and control channels. Fast dithering affects a quality of service (QoS) for a mobile phone in at least two ways. For one, the quality of audio carried in audio channel will deteriorate from a tone produced by periodic switching noise generated by a mobile phone's DC-to-DC converter having a switching signal undergoing fast dithering. Secondly, a call in progress via a mobile phone may be dropped due to a repeated corruption of a slow control channel word or byte due to EMI from switching noise generated by a mobile phone's DC-to-DC converter having a switching signal undergoing fast dithering.
An examination of information carried over a GSM network is helpful in evaluating other negative impacts of a fast dithering of a DC-to-DC converter's switching signal. FIG. 3 depicts a GSM Frame with Burst structures. There eight bursts in a GSM frame of a mobile phone call. One burst is used for reception and another burst is used for transmission. The remaining bursts may be used to by a mobile phone controller to monitor an adjacent cell's signal power along with other information in case a handover of a call is needed. A normal burst typically includes a traffic channel that can carry audio and/or other types of data including control information.
In a case involving audio data, the audio data is segmented into 20 ms segments and is usually encoded using a Regular-Pulse-Excited-Linear-Predictive-Coder (RPELCPC). Each of the 20 ms segments generates 260 coded bits, which are classified in three groups known as group 1a, group 1b and group II. Based on subjective testing, the three groups exhibit significant variations in sensitivity to errors. Errors within the first 50 coded bits in group 1a cannot be tolerated, whereas the coded bits of group II can tolerate some errors. However, the coded bits of group II are relatively tolerant to errors.
A low signal to noise ratio or poor link quality due to EMI from switching noise generated by a mobile phone's DC-to-DC converter having a switching signal undergoing fast dithering will introduce errors into the coded bits of groups 1a, 1b and II. Individual errors in the coded bits may be corrected by a decoding process, but blocks of errors in coded bits of groups 1a, 1b and II are relatively harder to correct. If any of the group 1a coded bits remain incorrect after the decoding process a frame erasure is likely to occur. As a result of multiple frame erasures, a higher than acceptable BER and/or FER will result in unacceptable poor audio quality, slow data rates, dropped calls and failures to respond to paging. Moreover, other mobile phones in their respective downlinks will be negatively affected by a mobile phone having a DC-to-DC converter with a switching signal undergoing fast dithering. Thus, it may be tempting to try a slow dither the switching signal of a DC-to-DC converter. However, experience has shown that whereas a fast dither of a switching signal results in an increase in BER relative to FER, a slow dither of a switching signal results in an increase in FER over BER. Thus, there remains a need to provide switching signal dithering circuit and method for a switching power supply such as a DC-to-DC converter for a mobile phone that will yield a low voltage ripple while reducing EMI and RF spurious transmissions. Moreover, there is a need for a switching signal dithering circuit and method that will result in less stringent filtering requirements, reduced cost, and increased efficiency for switching power supplies.